The present invention relates to a rotatable memory disk apparatus and, more particularly, to a rotatable memory disk apparatus which records data on tracks of the memory disk or reproduces the recorded data while the memory disk rotates.
A recent document filing system, for example, employs an optical disk apparatus. The optical disk apparatus is provided with a number of spiral or coaxial tracks. Data are recorded or the recorded data are reproduced on or from the tracks, while the optical disk rotates. For data recording, the filing system two-dimensionally optically scans a document to obtain optical image data thereof, converts the optical image data into corresponding electrical image data by photoelectric conversion means, and then optically records the electrical image data on the tracks of the optical memory disk. The recorded data is retrieved, as required, and is reproduced in the form of hard or soft copy. An optical head, which is disposed close to the surface of the optical memory disk, is used for recording or reproducing the electrical image data from the rotating optical memory disk. The optical head is mechanically coupled with a linear motor which is linearly driven in a radial direction of the optical disk. A position detector for detecting the position of the carriage for the linear motor, that is, the position of the optical head is provided. The position detector includes an optical linear scale system which includes a first lattice pattern movable together with the carriage and a second lattice pattern which is fixed on a stationary position and serves as an optical filter. The first and second lattice patterns are arranged between a light emitting element and a light detector in this order. While the position of the carriage or the position of the optical head is detected by the position detector, the linear motor is moved according to the detected position. Thus the optical head is positioned at an addressed track with an accuracy of 0.1 mm or less. The positioning means for the optical head is well-known and description thereof will be omitted. The outer tracks of the optical memory disk are longer than the inner tracks. For recording the data at a high density, therefore, the rotational speed of the optical memory disk for recording the data in the outer tracks must be slower than for recording the data in the inner tracks. To be more specific, in the recording and reproduction of data, the rotational speed of the optical memory disk must be controlled so that any given track always has a constant speed relative to the optical head. To this end, it is necessary to have means for changing the rotational speed of the optical memory disk according to the position of the addressed track on the optical memory disk. The conventional rotatable memory disk apparatus employs means to change the rotational speed of the optical memory disk in response to a signal representing the position of the head detected by the position detector. This approach, however, requires time until the rotational speed of the memory disk becomes stable, that is, the rotational speed corresponds to a final position of the head at which the addressed track is located. Also, during this period, the memory disk continues its rotation and the head also continues to flow up the addressed track. Therefore, in the case of spiral track, the position of the head during the time in which the head flows up the addressed track is shifted from the position predetermined for the addressed track. This means that data recording and data reproduction can not be conducted on an addressed track under the condition that the memory disk rotates in a rotational speed predetermined for the addressed track. Also, there is a case in which data is recorded on a memory disk by using a first disk apparatus and the data are reproduced by using a second disk apparatus. In such a case, if a difference of part fittings exists between the first and second disk apparatuses, the rotational speeds of the disk for the same track differs from each other in the first and second disk apparatuses. This gives a remarkably bad result during data reproduction.